A composite video signal is the sum of a luminance (brightness) signal and a chrominance (color) signal. These signals may be referred to as luma and chroma signals, respectively. The frequency ranges of the luma and chroma signals are designed to overlap. In video processing, the luma and chroma signal components are added together in order to generate a composite video signal. The luma and chroma video elements are integrated and broadcasted as a single composite video stream. Once the broadcasted composite signal is received, the luma and chroma signal components must be separated in order for the video signal to be processed and displayed. A comb filter may be utilized for separating the chroma and luma video signal components. For example, a television may be adapted to receive a composite video input and utilize an integrated comb filter to separate the chroma and luma video signal components. However, before the television can display the received video signal, the chroma and luma video components have to be separated.
FIG. 1 is a diagram illustrating generation of a conventional composite video signal. Referring to FIG. 1, a conventional composite video signal 105 may be generated from a luma component 103 and a chroma component 101. The composite video signal 105 may be generated by adding the chroma video signal component 101 and the luma video signal component 103. The chroma signal component 101 may be modulated at 3.58 MHz for a NTSC signal and 4.43 MHz for a PAL signal. The chroma signal component 101 may or may not comprise a constant chroma across the entire line. The luma signal component 103 may increase in amplitude in a stair step fashion or it may not.
FIG. 2A is a diagram illustrating modulated chroma signals in contiguous composite video frames in a NTSC video signal. The chroma component may be modulated so that a frequency of each successive line of video may be phase-shifted by 180 degrees with respect to the previous line. Referring to FIG. 2A, the previous frame 201 may comprise a previous line 203, a current line 205, and a next line 207. Similarly, the current frame 209 may comprise a previous line 211, a current line 213, and a next line 215. The current line 213 in the current frame 209 may be phase-shifted by 180 degrees from the previous line 211 in the current frame 209, as well as from the next line 215 in the current frame 209. Similarly, the current line 205 in the previous frame 201 may be phase-shifted by 180 degrees from the previous line 203 in the previous frame 201, as well as from the next line 207 in the previous frame 201. In addition, since frames in the contiguous composite video signal are at a frequency rate of 59.94 Hz, there may be a 180-degree phase shift between two adjacent frames, for example, the current frame 209 and the previous frame 201. Correspondingly, the current line 213 in the current frame may be 180 degrees phase-shifted from the current line 205 in the previous frame 201.
In conventional video processing, there are three ways to separate the luma and chroma video components in a NTSC video signal and these include combing horizontally, combing vertically, and combing temporally. During separation of the luma and chroma components, there are three bandwidth directions that may incur losses in the separation process and in the separated signal. Depending on the combing method that is utilized, the separated signal may have reduced vertical bandwidth, horizontal bandwidth, and/or temporal bandwidth.
The first way to separate the luma and chroma video components in a NTSC video signal is by horizontal combing. Horizontal combing may be accomplished by utilizing a notch filter, for example. Since the chroma signal component in a NTSC composite video signal may be modulated at 3.58 MHz, a notch filter set at 3.58 MHz may be utilized. Combing vertically may also be utilized to separate the luma and chroma video components. Combing vertically may be achieved in three different ways—the current line may be combed with the previous and the next line, the current line may be combed with the line just before it, or the current line may be combed with the line just after it. The vertical combing is performed spatially, which involves combing only within one field at a time and without any temporal combing.
During combing in the current frame 209, for example, if the current line 213 is added to the previous line 211, the chroma content may cancel out and two times the luma content may be obtained. On the other hand, if the previous line 211 is subtracted from the current line 213, the luma content may cancel out and two times the chroma content may be obtained. In this way, luma and chroma content may be separated from the composite video signal for further processing. However, vertical combing may result in a reduced vertical bandwidth.
A third way to comb a NTSC composite video signal is to comb temporally. Combing temporally comprises combing between two adjacent frames, for example, the current frame 209 and the previous frame 201. Further, temporal combing may be characterized by a reduced temporal bandwidth. Luma and chroma content may be separated by utilizing the same addition and subtraction method between a current line and a previous line as it was utilized with vertical combing.
FIG. 2B is a diagram illustrating modulated chroma signals in a composite video frame in a PAL video signal. The chroma component in a PAL video signal, for example, may be modulated so that a frequency of each successive line of video may be phase-shifted by 90 degrees with respect to the two lines previous. Referring to FIG. 2B, the current frame 221 may comprise two lines previous 223, a current line 225, and two lines next 227. The current line 225 in the current frame 221 may be phase-shifted by 180 degrees from the two lines previous 223 as well as from the two lines next 227 in the current frame 221. In this regard, the two lines previous 223 and the two lines next 227 are in phase.
In conventional video processing of a PAL video signal, there are two ways to separate the luma and chroma video components and these include combing horizontally and combing vertically. During separation of the luma and chroma components, there are two bandwidth directions that may incur losses in the separation process and in the separated signal. Depending on the combing method that is utilized, the separated signal may have reduced vertical bandwidth and/or horizontal bandwidth.
The first way to separate the luma and chroma video components in a PAL video signal is by horizontal combing. Horizontal combing may be accomplished by utilizing a notch filter, for example. Since the chroma signal component in a composite video signal may be modulated at 4.43 MHz, a notch filter set at 4.43 MHz may be utilized. Combing vertically may also be utilized to separate the luma and chroma video components in a PAL video signal. Combing vertically may be achieved in three different ways—the current line may be combed with two lines previous and two lines next, the current line may be combed with two lines previous, or the current line may be combed with two lines next. The vertical combing is performed spatially, which involves combing only within one field at a time and without any temporal combing.
During combing in the current frame 221, for example, if the current line 225 is added to two lines previous 223, the chroma content may cancel out and two times the luma content may be obtained. On the other hand, if two lines previous 223 is subtracted from the current line 225, the luma content may cancel out and two times the chroma content may be obtained. In this way, luma and chroma content may be separated from the PAL composite video signal for further processing. However, vertical combing in this manner may result in reduced vertical bandwidth.
FIG. 2C is a diagram illustrating combing of a correlated current line and a previous line in a current frame in a NTSC video signal. In this case, there is no vertical bandwidth and the previous line and the current line are perfectly correlated. The current line may be added with the previous line and two times luma may be obtained. Similarly, the previous line may be subtracted from the current line so that two times chroma may be obtained.
FIG. 2D is a diagram illustrating combing of a correlated current line and two lines previous in a current frame in a PAL video signal. In this case, there is no vertical bandwidth and the two lines previous and the current line are perfectly correlated. The current line may be added with the two lines previous and two times luma may be obtained. Similarly, the two lines previous may be subtracted from the current line so that two times chroma may be obtained.
FIG. 2E is a diagram illustrating combing of a non-correlated current line and a previous line in a current frame in a NTSC video signal. In this case, there may be significant vertical bandwidth. The vertical bandwidth may be high enough so that there may be no correlation between the current line and the previous line. When the current line and the previous line are combed together, there may be significant error in both the luma and chroma. This may produce combing artifacts in the obtained combed video signal. A substantially the same result may be obtained when combing temporally when there is temporal bandwidth, which indicates motion. Higher bandwidth in a given direction may cause combing in that direction to result in more incorrectly separated luma and chroma.
FIG. 2F is a diagram illustrating combing of a non-correlated current line and two lines previous in a current frame in a PAL video signal. In this case, there may be significant vertical bandwidth. The vertical bandwidth may be high enough so that there may be no correlation between the current line and the two lines previous. When the current line and the two lines previous are combed together, there may be significant error in both the luma and chroma. This may produce combing artifacts in the obtained combed PAL video signal. Higher bandwidth in a given direction may cause combing in that direction to result in more incorrectly separated luma and chroma.
During combing of a PAL or NTSC composite video signal, a luma component may be incorrectly determined to be chroma. As a result of the incorrectly characterized luma, a resulting combed PAL or NTSC video signal may look soft with insufficient sharpness and crispness.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.